How and when will I learn whether I'm carrying twins or more?

Decades ago, most women who were pregnant with multiples didn't find out until they were in labor. But such late-term surprises are rare today. Women typically discover they're having more than one baby during an ultrasound, often in the first trimester.

If you're pregnant as a result of a fertility treatment such as Clomid, gonadotropins, or in vitro fertilization (IVF), you'll probably have an ultrasound within the first eight weeks to count the number of embryos that have implanted. Ultrasound is almost foolproof at revealing multiple pregnancies, particularly after six to eight weeks. However, the more babies you're carrying, the easier it is for one to get overlooked.

How do twins end up fraternal or identical?

Fraternal twins develop from two eggs that are released and fertilized at the same time by two different sperm. They're known as dizygotic (DZ). Identical twins develop from one egg that splits into two and are thus known as monozygotic (MZ).

By the way, since they come from the same egg, monozygotic twins do share the same DNA, but these so-called "identical" twins are never exactly the same. Although they have the same genetic makeup, they may have distinguishing characteristics that allow their parents to tell them apart. This may be due to environmental influences, either inside or outside the womb. Also, certain genes may end up being expressed differently in each twin.

How and when can I find out whether my babies are dizygotic or monozygotic?

An ultrasound can often tell you whether your babies are dizygotic or monozygotic, based on whether there are one or two placentas and whether the babies are the same gender.

An experienced technician performing a transvaginal ultrasound between 9 and 14 weeks can determine with nearly 100 percent certainty whether your babies share a single placenta. (The accuracy rate drops to about 90 percent in the second trimester as the womb becomes more crowded.)

If your babies share a placenta, they're monozygotic. If there are two placentas, your twins may be DZ or MZ. All dyzygotic twins and 20 to 30 percent of monozygotic twins have separate placentas.

By 18 to 20 weeks, a technician may be able to identify the babies' genders, assuming that both babies are positioned so that the technician can get a good look at their genitals. If an ultrasound clearly shows that you have a boy and a girl, you'll know that your babies are dizygotic. Monozygotic twins are nearly always the same sex.

If the ultrasound shows two placentas and only one gender—or if the results are unclear—you may have to wait until your babies are born for your answer. After the birth, your practitioner will determine whether the twins shared a placenta. Because separate placentas sometimes fuse together and appear as one, a laboratory test may be needed to tell how many placentas there were.

If placental analysis doesn't solve the mystery, you can order an at-home DNA test for about $100, and get results in one or two weeks (or $400 with results in three days). Monozygotic twins will almost always have identical DNA, while dizygotic twins will share about 50 percent of their DNA.

As your babies grow up, it'll probably be easy to tell what type of twins they are just by looking at them. If they look so much alike that other people can't tell them apart, they're almost certainly monozygotic. Any obvious difference in their hair color, eye color, or facial features means that they're dizygotic.

Why is it important to know whether twins are dizygotic or monozygotic?

Finding out whether twins are DZ or MZ isn't simply a matter of satisfying idle curiosity. It can also be important to know for medical reasons.

For one thing, monozygotic twins who share a single placenta face a special health risk during pregnancy. Up to 15 percent of these twins will develop twin-to-twin transfusion syndrome (TTTS), a dangerous condition in which blood flows more to one twin than to the other. The twin on the receiving end tends to grow more rapidly than the donor twin.

Without treatment, the condition is usually fatal for both babies. If your twins are sharing a placenta, your caregiver will monitor their weight gain and watch for other signs of TTTS. If your caregiver suspects TTTS, prompt treatment may prevent complications and keep your babies healthy.

Even after your babies have grown up, it could be important to know whether they're dizygotic or monozygotic. For example, if one twin is found to have a genetic disease, an MZ sibling will have it, too. A DZ sibling, on the other hand, might be spared.

There's one more reason to find out whether your twins are fraternal or identical: Everyone will ask—from friends and relatives to strangers on the street. People are fascinated by twins, so be prepared to answer the question over and over again throughout your pregnancy and for years to come.

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